Sedimentation field flow fractionation is a versatile technique for the high resolution separation of a wide variety of particulates suspended in a fluid medium. The particulates include macromolecules in the 10.sup.5 to the 10.sup.13 molecular weight (0.001 to 1 .mu.m) range, colloids, particles, micelles, organelles and the like. The technique is more explicitly described in U.S. Pat. No. 3,449,938, issued June 17, 1969 to John C. Giddings and U.S. Pat. No. 3,523,610, issued Aug. 11, 1970 to Edward M. Purcell and Howard C. Berg.
In sedimentation field flow fractionation (SFFF), use is made of a centrifuge. A thin annular belt-like channel is made to rotate about the axis of the annulus. The resultant centrifugal force causes sample components of higher density than the mobile phase to sediment toward the outer wall of the channel. For equal particle density, because of their higher diffusion rate, smaller particulates will accumulate into a thicker layer against the outer wall than will larger particulates. On the average, therefore, larger particulates are forced closer to the outer wall.
If now the mobile phase or solvent is fed continuously from one end of the channel, it carries the sample components through the channel for later detection at the outlet of the channel. Because of the shape of the laminar velocity profile within the channel and the placement of particulates in that profile, solvent flow causes smaller particulates to elute first, followed by elution of components in the order of ascending particulate mass.
There are many criteria that a channel should meet in order to provide accurate particulate characterization data in short time periods. One such criteria is that the separating channel must be relatively thin. Unfortunately, this creates many problems in that the walls of the channel also should have a microscopically smooth finish to prevent the particles from sticking to the walls or being trapped in wall crevices. To provide such a microfinish, as well as to permit cleaning of the channel walls, it is desirable to have access to the interior of the channel. This is most easily achieved, as described in the Grant patent or the Romanauskas application by the use of mating inner and outer rings with a rectangular groove in the face of one or the other rings defining the channel.
A problem encountered when the channel is formed by mating rings is that of leakage. Leakage is caused by the centrifugally induced pressure inside the channel tending to force the fluid medium out between the contacting sealing surfaces of the rings. Leaks may occur because the high force field needed for the separation of the smaller particulates and lower molecular weight solutes distorts the channel itself and tends to cause leakage where none would normally exist. Another problem encountered in SFFF is the inability to easily provide a variety of channels having different widths, thicknesses, lengths, aspect ratios, and the like while maintaining the thickness dimension of the channel absolutely constant during centrifugal operation.